The transition of ethanol flames from conventional to MILD combustion

The present paper is focused on prevaporised ethanol flames in the transition from conventional combustion to the MILD combustion regime. Photographs and imaging of OH* chemiluminescence reveal a distinctive flame structure when ethanol carried by air burns in a 3% O2 coflow (typical of MILD combustion), differing from that at higher oxygen levels. In comparison to flames carried by air in a 9% O2 coflow, the spatial gradient and the peak in the OH* signal profile are significantly reduced in the 3% O2 coflow, indicating a more uniform distribution of heat release and temperature. The use of N2 as a carrier gas renders the OH* profile for the 6% O2 coflow case similar to that of flames carried by air in the 3% O2 coflow. The experimental results indicate a transition from conventional combustion to MILD combustion with the decrease of coflow O2 level and/or the use of N2 as a carrier gas. Calculations reveal that a substantial drop in the peak heat release rate and/or overall net heat release rate might contribute to the lack of luminosity of flames in the 3% O2 coflow, suggesting a need for threshold values of these two in defining MILD combustion. A series of laminar flame calculations are performed to identify the MILD combustion regime based on the absence of a negative heat release region. The absence of a negative heat release region is found to be strain rate dependent at a given temperature and O2 level of the oxidant stream. This is mainly a result of the enhanced transportation of O2 across the reaction zone at a higher strain rate. At low strain rates, the negative heat release region is more likely to disappear in a 3% O2 oxidant flow due to a combination of low flame temperature and high availability of O2.